The invention relates to the improved optical performance of light fixtures and optical elements, systems and other light-emitting devices whose primary function is to illuminate a region. Light fixtures are also commonly referred to as luminaires and represent a complete lighting unit consisting of lamp(s) and electrical controls (when applicable), together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply. Within the broad field of illumination, a category of light fixtures exists designed for applications commonly referred to as architectural lighting. Applications within the scope of the architectural lighting category include but are not limited to building façade lighting, wall washing, wall grazing, architectural flood lighting, architectural accent lighting, interior large-area lighting, interior accent lighting, back lighting, spot lighting, cove lighting, gallery lighting, landscape lighting, tree lighting, structure and bridge lighting, sculpture lighting, and urban landscape lighting. The invention is particularly well suited to architectural lighting and other applications where control of light output is important in illuminating objects by spreading substantially uniform light onto a surface over a distance.
LED Based Luminaires
Within the category of architectural lighting fixtures, solid-state lighting (SSL) or light-emitting diode (LED) fixtures utilize LEDs or organic LEDs (OLEDs) as light sources. The advantages of LEDs as light sources are well documented and include long life, high efficacy, and emission of narrow band colored light. For optical control over the light, the small area source of the light flux allows for the greater degree of control (more efficient control) over the light than that from fluorescent or extended light sources. Additionally, LEDs can be dimmed or cycled off and on without significantly adversely effecting performance or lifetime. Red, green, and blue LEDs can be connected with electronic controllers to adjust light output and provide a dynamic or adjusting color changing light output from a fixture.
Secondary Optics
The output from LEDs and LEDs plus components are typically near-Lambertian in angular luminous intensity output or have primary or secondary optics that reduce the angular intensity profile to a more narrow symmetrical profile. In near-Lambertian luminaries, the angular intensity profile typically distributes light to angular regions where it is not desired or needed, such as the high angles. This effectively reduces the efficiency because of the loss of light directed into the un-desired angular directions. In order to increase the angular intensity within a desired angular range, typical LED architectural lighting fixtures utilize secondary optic lenses that collimate light in narrow (such as a Full-Width at Half Maximum (FWHM) angular intensity of 12 degrees) symmetric beam patterns with an increased light flux within an angular range close to the normal to the plane of emission (optical axis) from the LED. The narrow symmetric beam patterns created by the angular light redistribution (collimation) through reflection from metallic surfaces or total internal reflections plus refraction typically have significantly higher light intensities in the central angular regions, such as a Gaussian-like light intensity distribution. These collimating optics are designed to provide a reduced angular distribution (more light closer to the zero illumination angle) and typically result in non-uniform illuminance from the non-uniform angular intensity.
Custom Secondary Optics
Secondary optics, such as reflectors or lenses or refractors, positioned in close proximity to the LEDs can be designed to provide a more custom, uniform light intensity distribution. However, the optics would need to be very specific to the application; and they are costly to manufacture, requiring custom molds that would likely need to be changed for each type of luminaire and each type of LED. Additionally, depending on the location of the LED in the array within the luminaire, the intensity distribution may need to be customized for each position within the array such that the light distribution from each LED overlaps to create a more uniform color and luminance
Light Directed Normal to the Primary Plane of Illumination
When typical LED architectural lighting fixtures are directed at an angle normal to a planar primary illumination surface such as a wall, the illuminance is non-uniform, decreasing rapidly from the central region. For example, an overhead luminaire with an array of LEDs with collimating secondary optics directed downward (normal to the floor) will non-uniformly illuminate the floor by directing more light normal to the floor and less to the outer angles with a significant, non-uniform fall-off in illuminance.
Light Directed at an Angle to the Primary Plane of Illumination
When luminaries are directed at an angle with respect to a primary illumination plane, the uniformity is not efficient due to the illuminance fall-off from the Cosine Law and the Inverse Square Law. The Cosine Law dictates that the illuminance over the angle for planar surfaces decreases as 1/cosine (theta) from the angular spread where theta is the angle from the luminaire or light source to the incidence plane normal. The Inverse Square Law states that the illuminance falls off by 1/r2 from the distance (r) between the light source to the illumination plane. Both of these contribute to the non-uniformity of the illumination, and large angles and longer distances from the source create a large illuminance non-uniformity.
Ideal Illuminance Uniformity
The illuminance spatial profile with the highest optical efficiency is a flat top, or step, illuminance profile such that the illuminance is constant across the area of interest. In order to achieve high levels of illuminance uniformity when the primary plane of incidence is at an angle with respect to the fixture, the angular light intensity distribution should pre-compensate for the specific installation or environment. This would require expensive custom tooled and molded optical elements for each specific application and condition.
Symmetric Spread Lenses
Other lenses or diffusers are sometimes added that are symmetrically refractive or diffusive such that the angular spread of the light is increased equally along two primary orthogonal axes. This can increase the illumination uniformity, but often at the expense of directing light into unwanted regions, thus reducing the illuminance in a desired region. Typically symmetric spread lenses are molded glass plates with prism patterns on at least one surface to refract light. The use of a symmetric spread lens will convert the spot illumination into a flood illumination with increased uniformity and reduced overall illuminance in the desired regions.
Asymmetric Spread Lenses
Some architectural lighting fixtures utilize asymmetric spread lenses to spread light asymmetrically such that the angular intensity along two primary orthogonal axes falls off more rapidly in one than the other. Asymmetric spread lenses, such as linear prism arrays, can be used to refract light asymmetrically, thus spreading the light more efficiently than symmetric spread lenses; however, the illuminance uniformity is still low because the output from the spread lens does not pre-compensate to account for the Cosine Law and the Inverse Square Law.
Conventional architectural lighting fixtures are far from optimal in effectively and efficiently distributing light onto target surfaces. Design tradeoffs associated with using conventional light fixture optical components make the desirable combination of wide beam spread, long optical throw, and substantially uniform distribution unobtainable. There is a need for even small improvements in further optimizing these parameters as well as luminaire efficiency. Grierson (U.S. Pat. No. 5,727,870) discloses using curved reflectors to achieve uniformity and Engel (U.S. Pat. No. 5,685,633) utilizes an asymmetric reflector to control light distribution. These reflectors require a significant amount of customization to enable the reflector to provide the often unique desired light output for a specific application and light source. Also, the illustrated metalized versions are costly to manufacture. The cost of customizing the shape and reflective properties can be very high. With the trend toward solid-state lighting where multiple LED light sources are needed, reflector geometry can get very complicated and occupy a significant amount of space behind and surrounding the light source. The efficiency and design of the system will often require each reflector to take into account the light profile from the others. Reibling (U.S. Pat. No. 4,188,657) discloses light fixtures with reflectors with different surface finishes to achieve non-symmetric output. These surface finishes are usually embossed, stamped, cast or formed into metal and are very costly to manufacture. Additionally, these features are prone to scratches during assembly or in uses and minor amounts of debris will significantly reduce their performance and cleaning is cumbersome due to particles trapped within the recesses. Wang (US 2006/0082989) proposes an array of LEDs positioned asymmetrically in order to achieve a more uniform light distribution when directing light toward an angled plane such as a wall. This arrangement has the disadvantage that the total area of the array is increased this results in a larger and more costly light fixture. Also, it does not account for the non-uniformity due to the Cosine Law and the Inverse Square Law. Rizkin et al (US 2004/0114355) describes the use of an LED array with collimation lenses to achieve a wider light distribution due to the distribution lengthwise of the LEDs. Similar to Wang, this geometry has the restriction that the array is longer in one dimension than the other and the total array of light emission is increased and the light fixture is increased in size. Harvey (US 2005018428) describes a reflector designed to asymmetrical direct light onto a panel. This uses one or more complex reflectors that are costly to produce and are custom for each application. Wijbenga et al. (U.S. Pat. No. 6,568,835) describes the use of facetted parabolic reflectors to generate an asymmetric light profile. These are also very customized and expensive to produce.